The present invention relates to a leakage detection circuit for use in a power source device provided with an electric motor vehicle, and more particularly to a leakage detection circuit for use in a power source device comprising a plurality of cells.
Electric motor vehicles are recently provided with a cell unit serving as a power source for a drive motor and generating a high voltage of at least 240 V. The cell unit has mounted an insulation member between the unit and a vehicle body, and is fixed to the vehicle body in a state of floating as electrically separated in order to avoid electrical shock to humans. The cell unit becomes high in voltage as described above, encountering the problem of electric leakage i.e., short-circuit accident between the cell and the vehicle body.
For example, electrolyte of the cell leaks out or, dust clinging to a surface of the cell, etc. is doused with water ingressing into the surface while the vehicle is driven in rainy weather, to impair insulation properties of the insulation member, causing such insulation fault that weak leak current flows, thereby impressing a high voltage of the cell unit on the vehicle body. This increases hazard including an electric shock accident caused by human contact to the vehicle body, and spark occurrence caused by large current discharging with the contact of an electrical conductive tool, etc.
It is conventional practice to detect electric leakage by providing a leakage detection circuit shown in FIG. 6. With the leakage detection circuit, a pair of resistors 5, 6 are connected to opposite ends of a cell unit 1, respectively. An intermediate point between the resistors 5, 6 is grounded to the grounding (vehicle body) via a resistor 2 for detecting electric leakage.
The cell unit 1 comprises a plurality of secondary cells such as nickel hydrogen cells which are connected to one another in series, which is likely to cause breakdown between a point of interconnecting secondary cells and the ground. The breakdown occurrence at such a position entails a problem of creating a dead zone wherein the leakage cannot be detected, or of reducing detection sensitivity as will be described below.
As shown in FIG. 6, a resistor 4 corresponds to breakdown occurrence at an intermediate point of the cell unit 1. Current flows through a leakage detection resistor 2 by way of 2 routes as indicated by an arrow in a solid line and by an arrow in a broken line. Suppose when the currents are i1, i2 as illustrated, the currents are expressed as follows:
i1=(B/2)/(Rs+R1+R)
i2=(B/2)/(Rs+R2+R)
In the case where a value of resistance R1 for the resistor 5 is equal to a value of resistance R2 for the resistor 6, the two currents, i1 and i2 are equal in amount and are opposite in flowing direction, so that a voltage V1 detected is zero in spite of electric leakage occurrence. Even if a value of resistance for the resistor 5 is different from a value for the resistor 6, the dead zone will be created when leakage occurs at any point of connection between a plurality of cells constituting the cell unit 1.
In this case, the currents i1 and i2 flow in opposite directions each other, as described above, so that a voltage V1 detected becomes a smaller value, making sensitivity reduced, making it difficult to detect the voltage. Furthermore, when a voltage (+B) of the cell unit 1 varies, the voltage changes a detection value of leakage, whereby entailing a problem that the leakage cannot be detected with high accuracy.
An object of the present invention is to provide a leakage detection circuit for use in a power source device comprising a cell unit of high voltage, which circuit is adapted to detect reliably with a simple structure leakage occurrence in a cell unit, and which is adapted to presume a portion of leakage.
A leakage detection circuit for use in a power source device embodying the present invention comprises:
a first current path being connected to opposite electrodes of the cell unit comprising a plurality of cells, and having a reference point generating a reference voltage corresponding to potential difference between the opposite electrodes,
a second current path being connected to the opposite electrodes of the cell unit, and having three points which are different in potential difference, the three points of which an intermediate point is connected to a grounding via an insulation resistor,
a first and second comparators having one input end to which voltage is applied from each of the two points divided by the intermediate point of the second current path, and having the other input end to which reference voltage is applied from the reference point of the first current path, and
a detection circuit detecting leakage occurrence based on outputs of the first and the second comparators.
With the leakage detection circuit of the invention described, suppose the leakage occurs at any point of connection between the plurality of cells constituting the cell unit. There is no change in current flowing through the first current path, and the reference voltage generated at the reference point is constant. On the other hand, in the second current path, leaking current flows from the intermediate point through the insulation resistor to the grounding (vehicle body), generating change in potential of the two points divided by the intermediate point, whereby outputs of the first and the second comparators are changed. As a result, the leakage occurrence is detected.
Accordingly, even if leakage voltage varies along with magnitude of leakage current, the leakage can be detected reliably since the reference voltage is fixed.
Stated specifically, a circuit constant of the first and the second current paths is so adjusted that in the event of leakage occurrence at any point of connection between the cells constituting the cell unit, a dead zone in detecting leakage based on the outputs given by the first and the second comparators is contained in an inside of potential distribution region corresponding to one cell, which is included in potential distribution generated between opposite electrodes of the cell unit.
This eliminates the problem of the dead zone in the leakage detection.
A leakage detection circuit for use in a power source unit of the invention comprises:
a first current line having opposite ends connected to opposite electrodes, respectively, of the cell unit, and having connected in series each other first and second voltage dividing resistors, and having the resistors interposed between the opposite ends,
a second current line having opposite ends connected to opposite electrodes, respectively, of the cell unit, and having connected in series sequentially a first protection resistor, a first detection resistor, a second detection resistor, and a second protection resistor, and having the resistors interposed between the opposite ends,
a grounding connection line connecting to a grounding via an insulation resistor a point of connection between the first detection resistor and the second detection resistor which are interposed on the second current line,
a first comparator having one input end connected to a point of connection between the first protection resistor and the first detection resistor which are interposed on the second current line, and having the other input end connected to a point of connection between the first voltage dividing resistor and the second voltage dividing resistor which are interposed on the first current line,
a second comparator having one input end connected to a point of connection between the second detection resistor and the second protection resistor which are interposed on the second current line, and having the other input end connected to a point of connection between the first voltage dividing resistor and the second voltage dividing resistor which are interposed on the first current line, and
a detection circuit detecting leakage occurrence based on outputs of the first and the second comparators.
With the leakage detection circuit of the invention described, suppose the leakage occurs at any point of connection between the plurality of cells constituting the cell unit. There is no change in current flowing through the first current line, and a reference voltage generated at a point of connection (reference point) between the first and the second voltage dividing resistors is therefore constant. On the other hand, in the second current line, leaking current flows from a point of connection (intermediate point) between the first and the second detection resistors through the grounding connection line and the insulation resistor to the grounding (vehicle body), generating change corresponding to the magnitude of the leakage current in potentials of a point of connection between the first protection resistor and the first detection resistor and in potentials of a point of connection between the second detection resistor and the second protection resistor, whereby outputs of the first and the second comparators are changed. As a result, the leakage occurrence is detected.
Accordingly, even if a leakage voltage varies along with the magnitude of the leakage current, the leakage occurrence can be detected reliably since the reference voltage is fixed.
Stated specifically, the first and the second comparators each outputs two signals different in potential corresponding to magnitude relation between a voltage impressed to one input end and a reference voltage impressed to the other input end. For example, the first comparator outputs a signal xe2x80x9chighxe2x80x9d when the voltage to be impressed to the one input end is greater than the voltage to be impressed to the other input end. The second comparator outputs a signal xe2x80x9chighxe2x80x9d when the voltage to be impressed to the one input end is smaller than the voltage to be impressed to the other input end.
Stated further specifically, the detection circuit comprises a photocoupler which is connected to an output end of the first comparator and to an output end of the second comparator. The photocoupler comprises a light-emitting diode for emitting light corresponding to potentials of the output ends, and a phototransistor to be turned on by light-emitting of the light-emitting diode, and detects leakage occurrence based on on/off of the phototransistor.
Stated furthermore specifically, values of resistance for the first and the second voltage dividing resistors interposed on the first current line, for the first and the second protection resistors interposed on the second current line, and for the first and the second detection resistors interposed on the second current line are so adjusted that in the event of the leakage occurrence at any point of connection between the plurality of cells constituting the cell unit, the dead zone in detecting leakage, wherein a voltage to be impressed to the one input end of the first comparator is greater than a reference voltage to be impressed to the other input end, and a voltage to be impressed to one input end of the second comparator is smaller than a reference voltage to be impressed to the other input end, is contained in an inside of a potential distribution region corresponding to one cell, which is included in potential distribution generated between opposite electrodes of the cell unit.
Consequently, even in the event of the leakage occurrence at any point of connection between the plurality of cells comprising the cell unit, the potential at the point of the leakage occurrence does not correspond to the dead zone.
The leakage detection circuit for use in the power source device embodying the invention, as described above, the leakage occurrence at the point of connection between a plurality of cells comprising the cell unit can be reliably detected. This can forestall electric shock accidents.